Not OP, but I think the gist of it is that religion was a major wedge between different Europeans, and was therefore an obstacle to establishing a unified "whiteness". By separating church and state, no one form of Christianity was favored over another, so the Catholics and the Puritans and the Anglicans and so on wouldn’t be compelled to fight one another. We forget how many millions of Europeans were killed over different forms of Christianity, and removing that stumbling block allowed the ideology of the settler colony to establish the white citizen and the black or brown other. In Chapter 1 of Black Marxism, Robinson touches on how Christianity failed to provide a unified European identity after the Reformation, which ultimately led to the formation of nationalist and racial ideologies in its place.